Steel casting generally uses solidification of steel to form the rough shape of the final product. The liquid steel is poured into a prepared mold of bonded sand. The sand mold cavity forms the desired shape from the liquid steel, and as the steel cools and solidifies, the product is formed. The liquid steel is melted in a furnace and then transferred from the furnace to the mold via a ladle. A ladle, is typically a steel shell, shaped like a large cylinder and lined with a refractory to resist the high temperature of the liquid steel. Liquid steel can be poured over the top edge of the ladle in a manner analogus to a teapot or common pitcher. However, pouring large quantities of steel over the edge of the ladle is impractical. Bottom pouring ladles avoid dropping the steel from the lip of lip pour (or teapot) ladles and allows good control over the pouring operation and is therefore the pouring method of choice. In bottom-pour ladles, a valve is placed at the bottom to allow liquid metal to flow from the ladle to a mold positioned thereunder. The most common valve used in bottom pouring steel is a nozzle positioned in an aperture in the ladle bottom and kept closed by a stopper-rod that seals the nozzle opening.
The nozzle-stopper-rod combination is normally of simple design. The stopper-rod has a simple rounded end and is coupled to the ladle to enable the intermittent flow of liquid steel from the ladle to a mold or a series of molds. The rounded end of the rod is seated on a smooth sealing area above the entrance opening of the nozzle. Generally, the nozzle is of a simple shape that has a sealing or seating area for the rod and a simple circular exit hole. The stopper-rod is on an assembly which keeps it seated on the nozzle during normal operation but that can be lifted off of the seat to allow pouring. When the rod is lifted, the liquid steel is allowed to flow past the stopper-rod-nozzle seal area, through the exit hole of the nozzle and into the mold. However, in bottom-pour ladles, oxide particles formed in prior steel processing can attach to the nozzle sealing area, and thereby slow or even stop the flow of liquid steel through the nozzle.
When pouring liquid steel from the ladle, the liquid steel interacts with the air and can form additional oxide particles which may be detrimental in the final product. Conventional nozzle-stopper-rod combinations produce poor quality product as well as a poor quality exit stream of liquid steel in that the liquid steel entrains air and forms additional oxide particles. If the nozzle-stopper-rod combination is not fully opened, severe air entrainment and oxide formation results.
Conventional nozzles are made of clay bonded ganister or alumina graphite combinations. The oxides formed in prior steel processing are composed of strong oxide forming metals such as aluminum. Due to the thermodynamic properties of these materials, these oxide particles prefer to stick to the nozzle material rather than to remain in the liquid steel. The sticking of the existing oxide particles is the cause of nozzle blockage. Other refractory combinations of aggregate and binder avoid the sticking. Calcia nozzles have been used successfully to avoid nozzle blockage. The oxide particles of alumina form a liquid oxide layer when they attach to calcia thus preventing blockage. Nozzles made of silica also avoid the sticking of oxide particles by the formation of liquid oxide interaction layer.
Poor exit stream quality is common with conventional nozzle-stopper-rod systems. As the metal enters the nozzle area, the general turbulence and fluid flow characteristics impart horizontal and torsional velocity components to the exiting metal stream. The liquid metal moves across the ladle bottom toward the nozzle area and this is the origin of the horizontal component. The liquid metal begins to vortex in the nozzle area and this imparts a torsional component. The combination of the horizontal and torsional components causes the exiting metal stream to break-up during pouring. The exiting metal stream can be hollow or even umbrella shaped. This is particularly true when the nozzle-stopper-rod system is not completely open.
The existing flow control nozzles for bottom-pour ladles create defective castings, which increase the costs of the final product due to the costs of rectifying such defective castings. Our invention substantially reduces or eliminates the defective molds poured by means of bottom-pour ladles having conventional nozzle-stopper-rod systems.
Our invention incorporates a nozzle having a geometry that is designed to avoid the break-up of the exiting metal stream and still allow the use of the conventional stopper-rod. The entrance to the nozzle area has castellations which help to prevent the vortexing or torsional component of velocity. The seal area is identical to conventional nozzles to allow the use of the conventional stopper-rod. The exit of the nozzle is a tapered cross shaped section, a cruciform, that dampens the torsional and horizontal components of velocity. This results in a good quality exit stream of liquid metal when pouring and avoids nozzle blockage and/or clogging.